Studies are being carried out on the mechanisms of enzymes that catalyze hydrogen transfer, such as alcohol dehydrogenase, glutamate dehydrogenase, and uridine diphosphoglucose epimerase. Alcohol dehydrogenase has been shown to acquire the transferred hydrogen following incubation of the enzyme with labeled coenzyme and acid hydrolysis. The labeled species, previously thought to be tryptophan, appears to be a product of the reaction of a tryptophyl residue with NADH during acid hydrolysis, and studies are currently under way to determine the structure of the moiety that is labeled. An apparently identical product can be obtained from the acid catalyzed reaction of tryptophan or its derivatives with dihydronicotinamides. Uridine diphosphogalactose epimerase during reductive inactivation by sugar and UMP, has been shown to acquire labeled hydrogen from the 4 position of galactose, but not glucose. From the pattern of reductions we have developed a model of the required sugar structure necessary for interaction with NAD, namely a configuration of C-2, -3, and -4 identical to that in galactose. Other sugars that have been tested follow the predicitions of the model, and also the model has successfully predicted that myo-inosose-2, which contains a keto group at C-4, should oxidatively reactivate the enzyme.